The present invention relates generally to resistance welding and more particularly to a coated Hall effect sensor for measuring current flow in the welding apparatus.
Resistance welding is a process in which welding electrodes are placed in contact with a workpiece and then low-voltage, high-current pulsed electricity is made to flow between the electrodes and through the workpiece, causing intense heat in the workpiece which ultimately fuses the workpiece and causes a weld nugget to grow. The current is normally pulsed on for about one-third to one second and then off for typically one to twenty seconds, whereupon the cycle repeats. Conventionally, a current stepup transformer or welding transformer is used to develop the low-voltage, high-current electricity.
In order to control the quality of the weld and the rate of nugget growth, conventional resistance welding systems attempt to regulate the weld current by controlling the electrical energy fed into the primary of the welding transformer. The primary receives alternating current which is regulated by a phase-fired control circuit. A current sensor, responsive to either the primary or the secondary of the transformer, provides a feedback signal for controlling the firing angle of the control circuit to thereby control the weld current. Firing angles of between 70.degree. and 120.degree. are common. Typically, a resistance welding apparatus delivers welding current (secondary current) on the order of 5,000 to 30,000 amperes, with primary current in the 0 to 5,000 ampere range. To sense heavy current of this magnitude for feedback control purposes, a current transformer has traditionally been used. The current transformer is usually a wound toroidal coil for encircling the current carrying primary or secondary conductor. The current transformer is thus positioned in the magnetic field produced by the current flow and develops an induced current which is proportional to the magnetic field strength and, hence, proportional to the welding current.
One drawback of conventional current transformers is that they tend to be quite bulky and expensive, particularly those adapted for placement in the secondary circuit. The typical secondary circuit current transformer might measure four to twelve inches in diameter. Because they are bulky, current transformers are difficult to attach to multisecondary welding transformers or to welding transformers which are equipped with an integral welding gun. The integral welding gun and transformer package is highly desirable, since the heavy current carrying transformer secondary leads can be kept quite short, even where the control circuitry is at a remote location. Hence, the inability to work with these types of welding equipment is a serious shortcoming of the current transformer.
Another drawback of conventional current transformers is that they produce an output waveform which is proportional to the time derivative of the actual current waveform. This is an inherent property of inductive sensors of this type, which renders them inaccurate for use in phase-fired systems.
In nonwelding low current sensing applications, solid-state Hall effect devices have been employed to measure current. One could, for example, purchase a Hall effect device capable of measuring the comparatively small (by resistance welding standards) current flow in a household electric circuit. Hall effect devices are capable of being miniaturized and thus lend themselves well to confined space applications. However, Hall effect devices have heretofore been quite unreliable in resistance welding applications. In resistance welding applications, conventional Hall effect sensors have a very high failure rate; in field tests, Hall effect devices failed on an average of two per year per welding gun. The cause of this high failure rate has not heretofore been fully understood. In contrast, current transformers can operate for years without failure.
The inventor has discovered that a resistance welding apparatus radiates intense electromagnetic energy, primarily in the radio frequencies spectrum up to approximately 90 mHz., as a by-product of the welding operation, and that this energy may be the cause of high failure rate in Hall effect devices. The inventor has further discovered an extremely economical and highly effective solution to Hall effect device failure rate in the resistance welding environment.
The invention provides a sensor for measuring current in a current carrying component of a resistance welding apparatus comprising a Hall effect device for sensing the magnetic field produced by a current. The invention further provides a means for positioning the Hall effect device in the magnetic field of the current in a current carrying component of the resistance welding apparatus. A magnetically saturable or electromagnetically saturable coating is provided on the device for shielding the device from the intense electromagnetic energy produced as a by-product of the welding operation. The coating prevents the electromagnetic energy from damaging the Hall effect device and yet permits the device to measure current. The failure rate is greatly reduced. The magnetically saturable coating may be a ferromagnetic coating comprising a ferromagnetic material or mixture of ferromagnetic materials in powdered from suspended in a binder, such as epoxy. Mixtures of powdered nickel are particularly effective. The coating may be applied by dipping a commercially available Hall effect device in the coating mixture, or the coating may be fabricated as an integral part of the device.
According to the inventive method, the resistance welding current sensor is made by providing a Hall effect device, coating the device with a magnetically suturable coating, as by dipping, and positioning the coated device in a resistance welding apparatus where the device can receive magnetic energy from a current carrying component of the welding apparatus .